Positron emission tomography (PET) is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical such as .sup.18 F-fluorodeoxyglucose (FDG) is introduced into the body of a patient. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pair of 511 keV gamma rays which are detected and used to create a clinically useful image.
Traditionally, PET scanners have used detector elements arranged in circles or rings about the imaging region, with the plane of the rings perpendicular to the axis of the imaging region. Each ring corresponds to an axial slice of the product. As described in U.S. Pat. No. 5,210,420 to Hartz, axial septa made from a radiation attenuating material such as lead have been placed so that gamma radiation traveling at relatively large angles is absorbed by the septa prior to reaching the collimator. Data from each ring of a plurality of slices is reconstructed using a two dimensional reconstruction algorithm.
One disadvantage associated with traditional PET scanners is their cost and complexity. Because the detectors and associated septa define distinct axial rings, sampling in the axial direction is discontinuous. As a result, reconstruction is limited to a fixed slice thickness. A further disadvantage is that the septa produce a shadow-like sensitivity variation in the axial direction. This variation in sensitivity is not apparent in ring-type PET scanners because the axial sampling interval is equivalent to the septal spacing. In systems having increased axial resolution, this variation can lead to undesirable image artifacts.
The present invention addresses these shortcomings, and others. One advantage of the present invention is that gamma radiation having a relatively large axial angle can be absorbed prior to reaching the detector. Yet another advantage is that increased axial resolution may be obtained as compared to traditional systems. Similarly, image reconstruction into slices having various positions and widths is facilitated. Yet another advantage is that image artifacts introduced by axial septa can readily be reduced. More specifically, axial sensitivity variations can be reduced by selectively canceling artifact components having different frequencies. Still other advantages will be recognized by those skilled in the art upon reading and understanding the appended description.